1. Field of the Invention
The present invention relates to a method for forming a crystallized semiconductor layer which is crystallized from a non-single-crystal semiconductor layer by using laser beams, a method for manufacturing a semiconductor apparatus, an apparatus for forming a crystallized semiconductor layer, and a method for manufacturing a display apparatus.
2. Description of the Related Art
As well known, a thin film semiconductor device such as a thin film transistor (TFT) has a substrate in which a semiconductor layer consisting of a semiconductor substance such as silicon is formed on a base substance consisting of an insulating material such as quartz glass. In the semiconductor layer of this substrate, a channel area is defined between a source area and a drain area formed to be separated from each other. A gate electrode is provided on the channel area through an insulating film.
The semiconductor layer is generally formed of amorphous silicon or polycrystalline silicon. In a TFT using a substrate having a layer of amorphous silicon, a mobility of electrons or holes is very low. For example, since the electron mobility is usually (not more than approximately 1 cm2/V·sec), an operating speed of the TFT is slow. As a result, such a TFT in hardly be used in an apparatus which requires a high-speed operation.
A semiconductor layer in which a channel area is formed of a polycrystalline silicon film is recently in heavy usage for the purpose of increasing the mobility. This polycrystalline silicon film is composed of many crystal grains having a very small particle size. Therefore, when it is operated as a semiconductor circuit apparatus, crystal grain boundaries become an obstacle for a flow of electrons, and there is a limit in improvement of the mobility.
Therefore, there has been examined acquisition of a thin film semiconductor device with the mobility being increased by reducing or eliminating the crystal grain boundary in the channel area by increasing a size of the crystal grain of the polycrystal silicon film, and further reducing or eliminating an obstacle for an electron flow. For example, there has been attempted formation of a thin film TFT which realizes the mobility of approximately 100 cm2/V·sec by heating a polycrystal silicon film in a high-temperature furnace in order to increase a particle size and thereby growing crystal grains having a particle size of approximately 1 μm. In order to crystallize the amorphous silicon to have a large particle size and form a TFT in this manner, a heat treatment at a high temperature which is not less than 600° C. is required. Therefore, a quartz glass plate which can withstand a high temperature but is expensive must be used as an insulating substrate, and an inexpensive glass plate (e.g., a soda glass plate) cannot be used. Therefore, such a TFT becomes expensive and has a drawback that it is hard to be used in display apparatus or the like for a large-screen TV receiver which uses many TFTs.
Thus, there has been developed a method for crystallizing non-single-crystal silicon by a laser annealing process without using a high-temperature heat treatment step. For example, there have been proposed some attempts in which silicon of an amorphous silicon film or a polycrystal silicon film is crystallized or re-crystallized by irradiating the film with an excimer laser beam in order to obtain a polycrystal silicon layer composed of crystal grains with a large particle size, and they have been put into practical use. According to such methods, crystal grains can be increased in size even if an inexpensive glass plate is used as a substrate.
However, even in a crystallization method using an excimer laser beam or the like, a particle size of obtained crystal grains is approximately 1 μm at the maximum level, and the particle size is uneven (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-127301). This cited reference discloses a series of operations that a zonal amorphous silicon film is polycrystallized by a fusion re-crystallization method, an amorphous silicon film is further deposited thereon and this is crystallized by using a solid phase growth method. This prior art method is a technique which first forms a crystal of a zonal polycrystal film composed of many small crystal grains, grows a crystal in a horizontal direction by using this as a crystal seed in order to obtain a polycrystal film having a crystal with a large grain size.
Even this proposed method is not satisfactory. That is because an acquired maximum crystal grain size is approximately 1000 nm (i.e., 1 μm), the particle size is uneven and it is suggested that irregularities in mobility become large (see FIGS. 2 to 5 in the above-described cited reference).
Furthermore, as a problem which has passed unnoticed in the conventional polycrystal semiconductor device, there is a problem in a crystal grain arrangement conformation in a layer. That is, in the conventional polycrystal semiconductor layer, the crystal grain arrangement conformation in a two-dimensional direction is completely random, and aligning the crystal grains has not been attempted. The randomness of the crystal grain arrangement and the uneven particle size produce irregularities in characteristics of thin film transistors, and bring a serious drawback to performances of an apparatus to be used for the following reasons.
In an arrangement of a transistor circuit in a thin film semiconductor device, many unit circuits must be aligned regularly and systematically, e.g., in a geometrical arrangement conformation. If a crystal particle size or a crystal arrangement of a polycrystal layer which is a base of formation of circuits is uneven, unit circuits are formed over crystal grains with various particle sizes or arrangements. This brings a result that the mobility or the electron passage conformation differs depending on each unit circuit, and adversely affects performances of the thin film semiconductor apparatus. For example, if there are irregularities in characteristics of each unit circuit, the entire apparatus must be designed with low-level characteristics as a basis.